diag: return created error from diag_set()

And from diag_add(). This will be helpful not to bother with
box_error_last() and diag_last_error(diag_get()) in the future
patch. It will change some attributes of a just created
ER_READONLY error to add more details.

Part of #5568

src/lib/core/diag.h

@@ -347,7 +347,7 @@ struct error *
 BuildSocketError(const char *file, unsigned line, const char *socketname,
 		 const char *format, ...);
 
-#define diag_set_detailed(file, line, class, ...) do {			\
+#define diag_set_detailed(file, line, class, ...) ({			\
 	/* Preserve the original errno. */                              \
 	int save_errno = errno;                                         \
 	say_debug("%s at %s:%i", #class, file, line);			\
@@ -356,19 +356,21 @@ BuildSocketError(const char *file, unsigned line, const char *socketname,
 	diag_set_error(diag_get(), e);					\
 	/* Restore the errno which might have been reset.  */           \
 	errno = save_errno;                                             \
-} while (0)
+	e;								\
+})
 
 #define diag_set(...)							\
 	diag_set_detailed(__FILE__, __LINE__, __VA_ARGS__)
 
-#define diag_add(class, ...) do {					\
+#define diag_add(class, ...) ({						\
 	int save_errno = errno;						\
 	say_debug("%s at %s:%i", #class, __FILE__, __LINE__);		\
 	struct error *e;						\
 	e = Build##class(__FILE__, __LINE__, ##__VA_ARGS__);		\
 	diag_add_error(diag_get(), e);					\
 	errno = save_errno;						\
-} while (0)
+	e;								\
+})
 
 #if defined(__cplusplus)
 } /* extern "C" */